1. Field of the Invention
The present invention relates to solar cells and to a process for fabricating solar cells using unipolar microwave fields.
2. Description of the Prior Art
The utilization of solar energy as an alternative energy source has been limited by the lack of availability of efficient energy conversion mechanisms. Photovoltaic cells provide a means for directly converting solar energy to electricity. To make such solar cells economically advantageous, the "energy payback time" must be small. That is, the cumulative converted solar energy output from each cell must exceed, in a relatively short period of time, the amount of energy that was consumed in fabrication of the cell. One objective of the present invention is to provide a solar cell with relatively low energy payback time.
In the past, photovoltaic solar cells have been fabricated by using conventional diffusion techniques to form large area, shallow junctions in a single crystal semiconductor substrate. Typically, four inch diameter, semiconductor grade single crystal silicon wafers have been placed in a high temperature (1800.degree. F.) infrared or resistance heating furnace and subjected to an atmosphere appropriate phosphrous, boron or other dopant atoms. Alternatively, a dopant material has been sprayed or spun onto the wafer, and the coated wafer then placed in the same type of oven to accomplish diffusion and drive-in of the dopant atoms into the substrate.
In either case, very large amounts of energy are used to fabricate the cell. The wafers typically remain in the diffusion furnace for one-half to one hour for each diffusion step. However, the furnace itself must remain on for a far longer time, typically many hours, so as to ensure temperature stability. Moreover, wafer heating may be repeated several times during the fabrication of a single solar cell. Thus separate diffusion steps may be used to form the shallow junction on the front surface of the semiconductor wafer, and to form the different conductivity type back surface field diffusion on the obverse side of the wafer. Yet a third high temperature furnace step is used to sinter the requisite metallic conductors to the wafer. Each resultant solar cell must be operated for a long time before the cumulative recovered solar energy exceeds the quite large amount of energy that was used (primarily in the diffusion and annealing furnace steps) to fabricate the solar cell.
In addition to the large energy payback time, other shortcomings are inherent in prior art solar cells. For example, the theoretical efficiency of the cells may not be reached because of adverse effects resulting from heating of the entire silicon substrate in the furnace. The thermal stress in the silicon can produce deep lying traps resulting in greater leakage and shorter bulk lifetime of the silicon. This may result in reduced photovoltaic efficiiency, with a concomitant reduction in the conversion efficiency from solar to electrical energy.
Another shortcoming of this prior art technique is that it is impractical to compensate for variations from wafer to wafer. Usually, a number of wafers are placed in the furnace at the same time. Although subjected to the same temperature and atmosphere, differences often result in the diffusion depth, uniformity and/or dopant concentration. Individual correction is impractical. As a result, those devices not meeting specification must be eliminated, thereby reducing overall yield.
A further shortcoming of the prior art is that the solar cells must be fabricated of single crystal material. If a polycrystalline semiconductor is used, diffusion of dopant impurities into the gain boundaries converts these boundaries to conductors which short out the device junctions. Inoperative devices result.
Described differently, the polycrystalline semiconductor material can be visualized as containing multiple grains of single crystal silicon embedded in a matrix or separated by grain boundaries of non-single crystal silicon. When such a wafer is placed in a vapor diffusion furnace, the dopant atoms diffuse into each grain to a shallow depth, thereby forming the requisite junction. However, the same impurity atoms diffuse to a far greater depth, and to a greater concentration level within the grain boundaries. The boundaries thus become conductive, and either short out the junctions in the individual grains or act as a conduction path or sink to ground. When used as a solar cell, the photovoltaic current thus is shorted to ground and is not available as a useful output from the cell.
Another objective of the present invention is to provide a photovoltaic solar cell of high efficienty that may be fabricated from polycrystalline semiconductor material. Since such polycrystalline material generally is less expensive than a single crystal of like size, decreased cost results. Further, it may be possible to provide polycrystalline silicon wafers of significantly greater area than is possible with single crystal material.
Another objective of the present invention is to provide an improved metal annealing technique that is particularly useful for providing conductors on a solar cell. At present, the metal conductors are evaporated or electroless plated onto the face of the solar cell after junction formation. The cell is placed in a conventional infrared or resistance furnace to sinter the metal to the silicon. This increases power consumption during device fabrication, and may result in further degradation of the bulk lifetime. In addition, a separate furnace may be required for this purpose thereby increasing capital equipment requirements and the amount of floor space necessary for the production facilities. An object of the present invention is to eliminate such requirements by utilizing unipolar microwave energy to accomplish annealing or sintering of the metallic conductors. Advantageously, this is accomplished simultaneously with drive-in of the junction forming dopant atoms.
Still a further object of the present invention is to provide a technique for the annealing of ion implanted structures using microwaves.